It has been desired that wastes including municipal wastes, refuse-derived fuel (RDF), plastic wastes, waste FRP, biomass wastes, automobile wastes, waste oil, and the like are safely combusted to reduce the volume of the wastes, and the combustion heat of the wastes is effectively utilized. Because incineration ash usually contains harmful heavy metals, in order to reclaim the incineration ash, it is necessary to take some measures for stabilizing heavy metal components. Further, there has been a request for downsizing a waste treatment facility as a whole. In order to realize the above subjects, a gasification and slagging combustion system which can recover various metals and harmless slag capable of being effectively utilized, and recover energy in the form of heat, electric power or the like has been put to practical use. The gasification and slagging combustion system is not ordinary incineration, but is capable of performing material and thermal recycling.
In the gasification and slagging combustion system, wastes are pyrolyzed and gasified at a temperature of 450 to 750° C. to generate a gas, tar, char (solid carbon containing ash), and the like in the gasification furnace, and the generated gas and tar are introduced into the slagging combustion furnace together with particulate char and combusted by a secondary air at a high temperature under a low air ratio of about 1.3 to about 1.5 to increase a combustion temperature of a melting point of ash or higher (for example, 1300° C. to 1450° C.) in the slagging combustion furnace. In this high-temperature condition, molten ash is collected on a furnace wall surface and falls along the furnace wall surface, thus forming a flow of molten slag. This molten slag is contacted with cooling water to form water-quenched slag.
An ash melting system does not have a gasification furnace. In the ash melting system, ash is supplied directly to a melting furnace to produce molten slag. The process in which molten slag is converted into water-quenched slag is substantially the same as the above gasification and slagging combustion system. Therefore, details of the ash melting system will not be described here.
Next, a combination of a gasification apparatus comprising a fluidized-bed gasification furnace and a slagging combustion furnace comprising a swirling-type slagging combustion furnace will be described below. FIG. 1 shows a conventional melting system having a swirling-type slagging combustion furnace and a water quenching trough, and a slag separating apparatus for separating molten slag.
In FIG. 1, reference numeral 10 represents a swirling slagging combustion furnace (swirling-type slagging combustion furnace), reference numeral 30 represents a water quenching trough, and reference numeral 50 represents a slag separating apparatus. The swirling-type slagging combustion furnace 10 comprises a primary combustion chamber 11, a secondary combustion chamber 12, and a tertiary combustion chamber 13. A produced gas (combustible gas) 111 containing char and tar which has been generated by pyrolysis and gasification in a gasification furnace (not shown) is introduced into the upper part of the primary combustion chamber 11 tangentially to an inner wall surface of the primary combustion chamber 11. Then, the gas 111 is mixed with a gas for combustion (usually, preheated air) 115 introduced into the primary combustion chamber 11, and is combusted and flows into the secondary combustion chamber 12 where the gas is combusted at a high temperature of about 1300 to about 1450° C. Then, the gas flows into the tertiary combustion chamber 13 and is completely combusted, and resultant combustion exhaust gas 113 is then supplied to a waste heat boiler (not shown). In FIG. 1, reference numerals 15 and 16 represent a burner for startup and auxiliary heat supply of the slagging combustion furnace, respectively.
The generated gas 111 containing char and tar which has been introduced into the upper part of the primary combustion chamber 11 forms a swirling flow, and moves to the secondary combustion chamber 12 while it is combusted at a high temperature in the swirling flow. Under centrifugal forces by the swirling flow, ash contained in the char is changed to slag mists and collected on the furnace wall surface. The slag mists attached to the furnace wall surface form a layer of molten slag 121, and the molten slag 121 flows down the bottom of the secondary combustion chamber 12, then falls from a slag discharge port 14 onto the water quenching trough 30. Water for cooling molten slag (hereinafter referred to as slag cooling water 152) flows on the water quenching trough 30 at all times. The molten slag 121 which has fallen from the slag discharge port 14 is dropped into the slag cooling water 152 and quenched to form water-quenched slag 122. The water-quenched slag 122 flows together with the slag cooling water 152 into a water tank 51 of the slag separating apparatus 50. The water tank 51 has a slag settling function. The settled water-quenched slag is scraped and removed by scrapers 53 attached to a separating conveyor 52, and is carried upwardly and separated from the slag cooling water. The water-quenched slag 122 is then discharged from the separating conveyor 52 through a slag discharge opening 54 to the outside of the slag separating apparatus 50. The slag cooling water 152 in the water tank 51 is delivered by a pump 41 from the water tank 51 onto the water quenching trough 30 through a pipe-line 151 and a nozzle 32. The slag cooling water 152 is circulated and reused.
The slag discharge port 14 serves to discharge molten slag 121 from the swirling-type slagging combustion furnace 10. However, because the swirling-type slagging combustion furnace 10 is filled with the exhaust gas 112, the molten slag 121 accompanies the exhaust gas 112 which is unavoidably going to contact the slag cooling water 152. Since the exhaust gas 112 contains many components including harmful substances, the exhaust gas 112 tends to contaminate the slag cooling water 152 and deteriorates the quality of the slag cooling water 152 by being contacted with the slag cooling water 152. As a result, the recovered water-quenched slag 122 is contaminated by the slag cooling water 152.
When the high-temperature molten slag 121 contacts the slag cooling water 152, a part of the slag cooling water 152 is evaporated, and the generated steam goes up to cool the slag discharge port 14. Consequently, the molten slag 121 is liable to be solidified on the inner surface of the slag discharge port 14 and the portion around the slag discharge port 14, and in extreme cases, the slag discharge port 14 is clogged with the solidified slag.